1. Field of the Invention
The present invention relates to a switch provided with a terminal block, a cover coupled with the terminal block and a position detector therein, such as an inhibitor switch for electrically detecting a shift position of an automatic transmission, and a welding method of the switch.
2. Description of Related Art
Examples of inhibitor switches in accordance with related art are shown in cross sectional views of FIG. 9. As shown in FIG. 9, an inhibitor switch 101 is composed of a terminal block 103, a cover 105, and a moving block 107.
The terminal block 103 has a plurality of fixed contact points 109. The terminal block 103 is coupled with the cover 105 to form a switch housing 111. A contact accommodating chamber 113 is formed inside the switch housing 111. The terminal block 103 and the cover 105 are coupled in a thermal welding part 115 by, for example, ultrasonic welding.
In the thermal welding part 115, a share joint is used to ensure high welding strength. The share joint allows a circular projected rib 119 on the side of the cover 105 to be guidably fitted all around a circular welding groove 117 on the side of the terminal block 103 at the corresponding portion of the thermal welding part 115 before thermally welded, so that the axial centers of the terminal block 103 and the cover 105 may be aligned and the axial rotation direction thereof may be positioned around the axis of the moving block 107. The terminal block 103 and the cover 105 are vibrated by means of ultrasonic waves while being pressed against each other at this positioning state. As a result, the projected rib 119 and the welding groove 117 are thermally welded, achieving reliable ultrasonic welding.
In the ultrasonic welding, however, glass fibers 118 in a resin material develop a tendency to lie in parallel to a welding surface at welding due to ultrasonic vibrations, as shown in FIG. 10, leading to easy formation of partial gaps 120. For this reason, to ensure air-tightness inside of the switch housing 111, a packing 121 is required to be attached on the inner periphery of the thermal welding part 115.
Thus, the ultrasonic welding involves a step for assembling the deformable packing 121, making an automated assembly process difficult. Besides, no-good work pieces may be produced, for example, in fitting the packing 121. Yet, the press pressure between the terminal block 103 and the cover 105 during the assembly process becomes high because of use of the packing 121. This means that high energy is required during the ultrasonic welding step, which produces serious noise, placing pressure on field workers.
To avoid this problem, for example, it has been assumed that the cover 105 is formed of a laser transmissive material and the terminal block 103 is formed of a laser non-transmissive material, and a laser beam is irradiated from the side of the cover 105 onto the side of the terminal block 103 for thermal welding.
On the other hand, when the cover 105 is formed of a laser transmissive material alone, the cover 105 requires a given thickness to ensure the rigidity of the cover 105 itself and thereby, 40% or higher of transmission factor is needed to transmit the laser beam into the laser non-transmission material at the corresponding portion of the thermal welding part 115.
Accordingly, this method in accordance with the related art has a disadvantage in that even though the cover 105 is colored by an adjusted amount of pigment, a visible light passes through to the switch housing 111 and any dirt attached over time to the inner surface thereof and sparks generated when the switch is turned on/off become visible from the outside. This disadvantage is especially prominent in switches, which are required to turn on/off a large current, as in the case of an inhibitor switch for detecting the shift position of the automatic transmission of an automobile (refer to JP2002-192617A or U.S. Pat. No. 5,893,959).
As described above, the conventional switch has the following problem. When one of the cover and the terminal block is formed of a laser transmissive material alone and a laser beam is irradiated onto the other of them for thermal welding, the cover and the terminal block each are required to have a given thickness to ensure their rigidity. On the other hand, since the one of the cover and the terminal block requires a sufficiently high laser transmission rate to pass through a laser beam to the switch housing, the laser transmissive material is required to be colored for a visible light ray to pass through. In consequence, any dirt attached to the inner surface thereof and sparks generated in a structure equipped with contact points are inevitably visible from the outside.
In view of the above, there exists a need for a switch and a welding method of the switch which overcome the above mentioned problems in the conventional art. The present invention addresses this need in the conventional art as well as other needs, which will become apparent to those skilled in the art from this disclosure.